Standing wave electron beam accelerators have found wide usage in medical accelerators where the high energy electron beam is employed to generate x-rays for therapeutic and diagnostic purposes. In such applications, dosimetric accuracy at the level of 1% or better is highly desirable. Electron beams generated by an electron beam accelerator can also be used directly or indirectly to kill infectious agents and pests, to sterilize objects, to change physical properties of objects, and to perform testing and inspection of objects, such as containers, containers storing radioactive material, and concrete structures.
A critical problem in national security is inspection of cargo containers. Due to the potential consequences of a single container housing a weapon of mass destruction, 100% inspection of containers is highly desirable. Due to the high rate of arrival of such containers, 100% inspection requires rapid imaging of each container, which in turn, requires a high-pulse repetition frequency of 1000 Hz and higher. For such cargo inspection applications, discrimination against dense objects may require use of two energies, a high energy (“HI” mode) and a low energy (“LO” mode). Examples of Hi and LO modes include operation at nominal beam energies of 6 and 3 MV, and at 9 and 6 MV. Comparison of the images obtained in HI and in LO mode permits high-contrast inspection for and detection of dense objects, which may be indicative of a security threat.
Thus, applicant of the subject application recognizes that it may be desirable to have microwave power from a generator that varies between at least two power levels, such that an accelerator can generate charged particle pulses that vary between at least two different energy levels. However, applicant notices the following problems with existing power systems.
Existing power systems may not be able to accomplish stable and reliable pulse-to-pulse variation in output power. Also, existing power generators may not be able to provide generated power such that energy delivered to the accelerators can vary quickly, e.g., on the order of a millisecond, between at least two energy levels. This rapid variation may be desirable in certain ionizing-radiation systems, such as cargo inspection systems, and in certain medical systems, such as those use for treatment and imaging.
While it is possible to operate tubes with large variations in output power from pulse to pulse, there are certain disadvantages. For example, a magnetron based system may not perform stably when the high voltage pulse is changed by a large value from pulse to pulse. Also, a permanent magnet magnetron operated off of the constant load line may result in additional power dissipation in the modulator. Variation of magnetron frequency from pulse to pulse may not be practical due to mechanical limitations of the tuner, or stability issues associated with the magnetron. As a different example, a klystron-based system may not perform stably when the high voltage pulse is varied by a large value from pulse to pulse, particularly if the tube stability requirements favor operation at saturation. Finally, even where the tube is amenable to operation with a pulse-to-pulse variation in high-voltage, stability of the system as a whole may not be adequate for the application.
Further, in existing systems, microwave or radio-frequency (RF) power provided by a power generator to an accelerator may be reflected back to the power generator. In many applications, it is desirable to reduce this reflected power to a low value, thereby providing high isolation of the reflected power from the source. Sometimes, it may be desirable that such reflected power be controlled in phase and amplitude, so that the frequency of the power generator will be “pulled” to the accelerator frequency, resulting in a stable operation of the power generator and the accelerator. This is often the case for non-coaxial magnetrons. If the reflected power is not controlled, the frequency of the power generator will be pulled away from that of the accelerator, resulting in difficulty of getting the power generator to operate stably and reliably at the frequency that is optimal for accelerator's performance.